he 8600gt/gts are only 128bit, while the 7900/80-- are higher with 256bit to 320bit. 8600 are not worth buying
If you have around 250$ get the 8800gts 320mb which is better than the 8600 gt/gts, depending on your system but i guess at least 20amps and 500 watts, btw playing HD videos has nothing to do with a video card for games ...
You still have a CRT monitor? Holy cow, I thought those went extinct years ago.